Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/strip-ansi/index.d.ts

@@ -0,0 +1,15 @@
+/**
+Strip [ANSI escape codes](https://en.wikipedia.org/wiki/ANSI_escape_code) from a string.
+
+@example
+```
+import stripAnsi from 'strip-ansi';
+
+stripAnsi('\u001B[4mUnicorn\u001B[0m');
+//=> 'Unicorn'
+
+stripAnsi('\u001B]8;;https://github.com\u0007Click\u001B]8;;\u0007');
+//=> 'Click'
+```
+*/
+export default function stripAnsi(string: string): string;